Scanning electron microscope

ABSTRACT

With a scanning electron microscope (SEM) adopting a commonly available exhaust system such as a turbo-molecular pump, an ion pump, or a rotary pump, and so forth, there is realized an apparatus capable of safely executing observation, or adsorption of a target substance that is high in rarity. Further, there is realized a safe SEM low in the risk of an electrical discharge by providing the apparatus with a probe, a means for replacing an atmosphere in a specimen chamber, with a predetermined gas, and a means for forming an image by detection of an ion current, and detection of an absorption current. Further, there is provided a means for controlling the polarity of a voltage applied to the probe. Still further, there is provided a control means for controlling a value of the voltage applied to the probe according to a degree of vacuum.

TECHNICAL FIELD

The invention relates to a scanning electron microscope (SEM) suitableto observe, and extract a specimen in a particle-like shape, such as apowder, a particle, and so forth, and in particular, to a low vacuum SEMprovided with a function for observation of an absorption current image,and a probe capable of causing the specimen in the particle-like shapeto be adsorbed thereto.

BACKGROUND ART

There is available an apparatus capable of implementing variousfunctions by use of a probe. The probe is used for various applicationobjects; however, the probe is often used typically for an object ofmeasuring the volt-ampere characteristic of a transistor formed on asemiconductor integrated circuit, or for an object of causing a foreignmatter on a specimen under observation to be adsorbed to the probe forremoval. The probe described as above is made up so as to be freelymovable over a specimen or a specimen base by use of a predetermineddriving mechanism, the driving mechanism for the probe, as a whole,being called a probing system.

For example, in Patent document 1, there is disclosed an inspectionapparatus provided with a mechanism for removing a foreign matter on thesurface of a wafer or a photo mask by use of the probing system in asemiconductor inspection step. With the invention described in Patentdocument 1, the foreign matter is caused to be adsorbed to the probe bythe agency of an electrostatic force generated between the foreignmatter and the probe, and thereafter, the probe with the foreign matteradsorbed thereto is moved outside the surface of the wafer to be therebyremoved. Detection of a position of the foreign matter is made by use ofa laser-scanning microscope. Thereby, foreign matters one by one can bereliably removed while inhibiting damage on the surface of the wafer.

Further, in Patent document 2, there is disclosed a foreign matterremoval apparatus provided with an electron microscope, for removal of aforeign matter from a semiconductor wafer, and a stencil-mask,respectively. The foreign matter removal apparatus disclosed in Patentdocument 2 is provided with a canti-lever of an atomic force microscope(AFM), serving as a probe, whereupon a foreign matter is adsorbed to theprobe by the agency of a shear force occurring by causing thecanti-lever to be in contact with a foreign matter in a functionalregion (a region acting as a circuit) on a specimen, and the foreignmatter is irradiated with a laser beam, or a converging ion beam outsidethe functional region after the foreign matter is removed to the outsideof the functional region to thereby firmly fix, or remove the foreignmatter. For detection of a position of the foreign matter, use is madeof an SEM, or an optical microscope, and if the size of the foreignmatter is on the order of nanometer, the SEM is used, while if the sizeof the foreign is on the order of micrometer, the optical microscope isused. Because the bond strength of the foreign matter, by the agency ofthe shear force, is greater than electrostatic adsorption, even aforeign matter that is firmly fixed onto the specimen can be removedwith certainty.

CITATION LIST Patent Literature

-   Patent Document 1: Japanese Unexamined Patent Application    Publication No. Hei8(1996)-254817 (U.S. Pat. No. 5,634,230)-   Patent Document 2: Japanese Unexamined Patent Application    Publication No. 2005-311320

SUMMARY OF INVENTION Technical Problem

The probing system is used for various objects such as a object ofcausing a target substance to be adsorbed as above, or a object ofobserving a target substance in such a state as adsorbed to a probe, andso forth; however, the necessity of having the step of detecting theposition of the target substance by whatever method to thereby cause thetarget substance to be adsorbed to the probe is common to the variousobjects.

In the case of using the SEM for detection of the position of a targetsubstance, it is necessary that a specimen under observation is disposedin a high vacuum, and therefore, the specimen need be disposed in avacuum specimen chamber. A degree of vacuum inside the specimen chamberis dependent on a size of an observation specimen chamber, andperformance of an exhaust system (a vacuum pump), and in the case of acommon SEM, a degree of vacuum inside the observation specimen chamberis on the order of 10⁻⁴ Pa. In this connection, there is the case whereadsorption of the target substance to the probe need be carried out byprecluding occurrence of air pollution as much as possible owing tocharacteristic, or rarity of the target substance, and so forth. Inorder to remove a residual gas almost completely from the specimenchamber, it is necessary to raise the degree of vacuum inside theobservation specimen chamber to a very high level. For example, thedegree of vacuum of space is approximately on the order of 10⁻⁸ to 10⁻¹¹Pa, which is far above the degree of vacuum adopted in the observationspecimen chamber for use in the common SEM

A degree of vacuum attained in the specimen chamber is dependent on theperformance of the exhaust system, and a volume of the specimen chamber.However, in order to introduce the probing system in the specimenchamber, the specimen chamber needs to have dimensions on the order ofcertain values in magnitude, and therefore, if the residual gas is to bealmost completely removed from the specimen chamber, an extremelyhigh-performance, and expensive exhaust means or exhaust system will berequired.

In order to implement adsorption of the target substance to the probe byprecluding air pollution with the use of an inexpensive exhaust system,it need only be sufficient to cause an atmosphere in the specimenchamber to be replaced with an inert gas, such as nitrogen gas, argongas, and so forth. However, with the common SEM where secondaryelectrons are detected to thereby turn the secondary electrons into animage, there is the danger of occurrence of an electrical dischargebecause a high voltage is applied to the tip of a sintillator as asecondary electron detector. In the case where electrostatic adsorptionis adopted as a method for adsorption of the target substance, a voltageis applied to the probe, so that the danger of the electrical dischargesimilarly exists.

It is therefore an object of the invention to realize an apparatuscapable of safely executing observation, or adsorption of a targetsubstance, in an electron microscope adopting a commonly availableexhaust system such as a turbo-molecular pump, an ion pump, or a rotarypump, and so forth.

Solution to Problem

With the present invention, in order to solve the problem described asabove, there is provided a means for replacing an atmosphere in aspecimen chamber containing a target substance with a predetermined gas,and in order to acquire a specimen image in a low vacuum atmosphere,there is adopted an imaging system by detection of an ion current, andan absorption current. As a method for adsorbing the target substance,use may be made of either a method using an electrostatic adsorption, ora method using shearing stress. Thereby, observation of the targetsubstance is enabled in an SEM adopting a commonly available exhaustsystem.

Further, in the case where the target substance is rare, or fragile, andso forth, an electrostatic adsorption method that is an adsorptionmethod capable of soft adsorption is preferably used. With the presentinvention, it is possible to realize a scanning electron microscopecapable of implementing safe adsorption of a target substance even inthe case of adopting the electrostatic adsorption method.

Advantageous Effects of Invention

With the present invention, it is possible to provide an inexpensivescanning electron microscope capable of observing or adsorbing a targetsubstance without causing the danger of electrical discharge.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of an electrostatic adsorption mechanism for aprobe in a low vacuum scanning electron microscope;

FIG. 2 is a flow chart showing a process for electrostatic adsorption ofa particle to a probe;

FIG. 3 is a schematic representation showing a layout of two lengths ofprobe tips on an SEM screen;

FIG. 4 is a block diagram showing members of an interference-preventionmechanism, installed on a stage, and a prober mechanism, respectively;

FIG. 5 is a schematic representation showing the interference-preventionmechanism of FIG. 4, in as-activated state;

FIG. 6 is a functional diagram showing the interference-preventionmechanism in detail;

FIG. 7 is a block diagram of a control mechanism for automaticallycontrolling a voltage applied to the probe tips, and a degree of vacuum;

FIG. 8 is a schematic representation of the SEM screen, showing anoperation whereby if a particle is adsorbed to a site of a probe 19,other than the tip thereof, the particle is moved by use of anotherprobe; and

FIG. 9 is a layout top view of a specimen chamber in the case where twolengths of probers are provided.

DESCRIPTION OF EMBODIMENTS

Embodiments of a scanning electron microscope according to the inventionare described hereinafter with reference to the accompanying drawings.

FIG. 1 shows a general block diagram of a scanning electron microscope(an SEM) according to the present embodiment of the invention. The SEMaccording to the present embodiment is broadly comprised of a specimenchamber 1 for incorporating a specimen as a target substance forobservation, or adsorption, an electron optical tube 2 disposed abovethe specimen chamber 1, for irradiating the specimen with a primaryelectron beam, a detection system for detecting an ion current amplifiedin a gas to be converted into an image signal, an exhaust system forevacuating the specimen chamber 1 into a vacuum, a supply system forsupplying a replacement gas into the specimen chamber 1, a controlsystem for controlling an operation of an apparatus, as a whole, and soforth.

A specimen holder 4 with a target substance placed thereon, a stage 5for causing the specimen holder 4 to be moved, and a prober unit 7 forsupporting a probe to be adsorbed to the target substance are providedinside the specimen chamber 1. A specimen placement surface of thespecimen holder according to the present embodiment is made up of anelectrically conductive material, the specimen holder having a functionfor detecting an ion current described later on. The prober unit 7 isprovided on a prober base 10 in such a way as to be movable in auniaxial direction over the prober base 10. A prober mechanism system 14is made up of the prober unit 7, and the prober base 10. Aninterference-prevention mechanism 6 is provided on respective faces ofthe stage 5, and the prober base 10, opposing each other, detaileddescription thereof being given later on. The observation specimenchamber 1 is further provided with an infrared camera 9, the infraredcamera 9 being linked with a monitor 12 for the infrared camera, via acable. An image acquired by the infrared camera 9 is used to confirm astate of contact between the probe and the target substance.

The electron optical tube 2 is provided with a control means forirradiation with a primary electron beam, including an electron gun, acondenser lens, an objective lens, or a scanning deflector, the controlmeans causing the primary electron beam to scan the target substance onthe specimen holder 4 under a desired scanning condition. An ESEDelectrode 3 is provided at an end of the electron optical tube 2,adjacent to the target substance, the ESED electrode 3 being for use ingenerating an electric field for causing ions formed by collision ofsecondary electrons emitted upon the irradiation with the primaryelectron beam, against gas molecules in the surroundings, so as to bedrifted toward the specimen stage. The ions drifted toward the specimenstage are collected by the specimen holder 4 to be detected as the ioncurrent. Further, an orifice 42 is provided inside the electron opticaltube 2, thereby realizing differential pumping between the electronoptical tube 2 and the specimen chamber 1.

The detection system is made up of the specimen holder 4 provided withan ion current detection function, a first amplifier 8 for amplifying adetection current from the specimen holder 4, a movable typeback-scattering electron (BSE) detector 37, a second amplifier 38 foramplifying a detection current from the movable type BSE detector 37,and so forth. An output signal of the first amplifier 8 or the secondamplifier 38 is displayed on a monitor 13.

The exhaust system is made up of a first turbo-molecular pump 39 forevacuating the specimen chamber 1 into a vacuum, a secondturbo-molecular pump 41 for evacuating the electron optical tube 2 intoa vacuum, a first rotary pump 40 connected to the first turbo-molecularpump 39 and the second turbo-molecular pump 4, respectively to therebyfunction as a pump for backing up both the pumps, a second rotary pump44 for roughly evacuating the specimen chamber 1, a first valve 43 foropening/closing an evacuation path between the first turbo-molecularpump 39, and the specimen chamber 1, a second valve 45 foropening/closing an evacuation path between the second rotary pump 44 andthe specimen chamber 1, and so forth.

The supply system is provided with a gas-supply piping 36 for supplyingthe replacement gas into the specimen chamber 1. The gas-supply piping36 is connected to a gas-supply source (not shown) of a desiredreplacement gas. Further, the gas-supply piping 36 is provided with aneedle valve 11 for controlling a flow rate of a gas supplied to thespecimen chamber 1, the needle valve 11 being controlled by a controller31.

The control system is made up of a prober-unit power supply 15 forsupplying a prober drive current to be supplied to the prober unit 7,the controller 31 for controlling the needle valve 11, and theprober-unit power supply 15, and an operation PC 16 for setting anoperation condition of the apparatus, and so forth. The controller 31 ismade up so as to incorporate a digital-to-analog converter (DAC) forconverting respective digital control signals from a processor, and amemory, or a digital control signal from the processor, or a control PCinto an analog signal for controlling the needle valve 11, and theprober-unit power supply 15, respectively. The operation PC 16 isprovided with an input means (not shown) including a pointing device,such as a mouse, and so forth, a keyboard, and so forth, the operationPC 16 being capable of setting the operation condition of the apparatusvia a GUI displayed on the monitor 13. The controller 31, theprober-unit power supply 15, the needle valve 11, and the operation PC16 are connected to each other via a cable, respectively, and a signalis sent out from the operation PC 16 to the controller 31, whereupon theprober-unit power supply 15, and the needle valve 11 are controlled.

FIG. 2 is a flow chart showing a series of operations up to theelectrostatic adsorption of the target substance in the SEM according tothe present embodiment. Further, in the following description, aparticle placed on a suitable base material is described as the targetsubstance by way of example; however, it goes without saying that thetarget substance is not limited thereto.

In Step 1, a prober mechanism is installed to the specimen chamber of alow vacuum SEM. Upon installation of the prober mechanism, a probe isinstalled to the prober unit 7 at first, and further, the prober unit 7is installed to the prober base 10. Concurrently, the infrared camera 9is installed to the specimen chamber 1. An installation position of theinfrared camera 9 is a position in the specimen chamber 1 such that anirradiation position of the primary electron beam substantiallycorresponds to the center of a visual field of an infrared image.Thereby, an advance preparation is completed.

In Step 2, a specimen under observation is introduced into the specimenchamber, and vacuum drawing is executed inside the specimen chamber.When a degree of vacuum inside the specimen chamber is turned to on theorder of 10⁻⁴ Pa according to measurement by a vacuum gage, observationis enabled, and at this point in time, the needle valve 11 is opened,whereupon a gas (for example, nitrogen gas) is introduced into thespecimen chamber via the gas-supply piping 36. The needle valve 11 isunder control by the controller 31, and fine adjustment is made suchthat a degree of vacuum inside the specimen chamber 1 (a value in arange of 100 to 1000 Pa) matches a set value. The set value of thedegree of vacuum attained inside the specimen chamber 1 can be set bymeans of the GUI displayed on the monitor 13, and the operation PC 16.Upon attainment of the set degree of vacuum, the specimen holder 4 isintroduced into the specimen chamber. The stage 5 is moved such that aninitial position of the specimen holder 4 corresponds to a positionwhere the center of the specimen holder is located immediatelyunderneath the electron optical tube 2. In order to preclude airpollution, the specimen holder 4 is kept filled up with the gas whilethe top of the specimen holder 4 is covered with a special-purpose lid,and the lid is removed after the specimen holder 4 is introduced intothe specimen chamber. If this method for introducing the specimen holderinto the specimen chamber is adopted, it is possible to preventcontamination of the specimen holder 4 itself with an atmosphere beforeintroduction into the specimen chamber.

In step 3, rough adjustment of a position of the probe installed to theprober unit 7 is executed in three XYZ axial-directions. This move ismanually carried out; however, it is also possible to automate the moveby use of a motor. Adjustment of the position of the probe, in the XYplane, is made by use of the operation PC 16 operated by a user of theapparatus such that the probe comes into the visual field of an imageacquired by the infrared camera 9 while watching the image acquired bythe infrared camera 9. The image acquired by the infrared camera 9 isdisplayed on the monitor 12 for the infrared camera.

Upon the specimen holder 4 approaching the prober unit 7, a height ofthe stage 5 is adjusted such that the probe of the prober unit 7approaches a surface of the specimen holder 4 as much as possible. Asfine adjustment of the height of the stage 5, in the Z-axis direction,is made on the basis of an SEM image displayed on the monitor 13, aheight adjustment executed in the step 3 is a rough adjustment. Thereason for this is to prevent the probe from being collided with thespecimen holder 4 to be thereby damaged. In order to prevent the probefrom interfering with the specimen holder 4 to be thereby damaged inthis case, a travel distance of the prober unit 4, in the Z-axisdirection, and a distance (height) of the surface of the specimen arecalculated, and the controller 31 restricts a maximum travel distance ofthe prober mechanism travelling in the step 3 to a distance at which theprobe does not come into contact with the specimen.

In step 4, there is executed adjustment of the position of the probetoward the center of the SEM image. This is because only roughregistration is executed in the registration of the prober, by use ofthe infrared image, carried out in the step 3, and the respective tipsof plural probes are not located at the center of a visual field on theSEM image. Since the prober is movable with accuracy of micrometers, nodifficulty arises in implementing the registration of the probes towardthe center. In this operation, the user of the apparatus makes use of amanual operation; however, an automatic operation by use of thecontroller 31 is also possible.

If it can be confirmed that the prober unit 7 and the specimen holder 4approach each other within the image acquired by the infrared camera 9,the specimen is irradiated with an electron beam to thereby show an SEMimage on the monitor 13. Because the specimen chamber 1 is in a lowvacuum in this case, a common secondary electron detector where a highvoltage is applied is unusable owing to the danger of electricaldischarge. Accordingly, an ion current image from which an image like asecondary electron image is obtainable is picked up to be shown as animage on the monitor 13. The ion current attributable to the secondaryelectrons is detected by the specimen holder 4 to be amplified by theamplifier 8 before being displayed on the monitor 13.

In FIG. 3, there is shown an example of an SEM image in a state wheretwo lengths of probes have come into a visual field. In an operation foradsorption of a particle, plural probes are preferably used, making useof at least two lengths of the probes. For example, if a particle 30 isadsorbed to a site in a probe 19, deviate from the tip thereof, as shownin FIG. 8, this will interfere with smooth transportation of theparticle 30. If there are available plural lengths of probes (forexample, two lengths thereof), the particle 30 can be moved by use ofthe other probe 18. Accordingly, the number of probes is preferablyplural.

The prober mechanism system 14 is connected to a special-purpose powersupply (the prober-unit power supply 15) by a cable, and the probermechanism system 14 can be operated by use of the operation PC 16. Anoperator moves the prober unit 7 such that the probe comes to the centerof a visual field on the SEM image, as shown in FIG. 3. In the case ofthe two lengths of the probes, shown in FIG. 3, adjustment is made insuch a way as to prevent the tip 18 of the probe from interfering withthe tip 19 of the other probe on an SEM screen 17.

In step 5, there is executed fine adjustment of the height of the probe.This operation being for the purpose of the fine adjustment, the user ofthe apparatus executes the operation through a manual operation. In thestep 5, the probe is kept floated so as to be focused within a workingdistance (WD) in the SEM image while preventing the probe from cominginto contact with the specimen.

In step 6, the stage is moved to search a target particle. The probe isalways kept at the center of an image by moving the stage instead a sideadjacent to the prober unit 7, so that adsorption can be alwaysimplemented at the center position in the visual field of the SEM image(that is, the center position of an optical axis).

In this connection, an interference between the prober unit 7 and thespecimen holder 4 (or the stage 5), occurring at the time when the stage5 is moved, that is, a collision therebetween will raise a problem. Inorder to avoid the problem, a maximum travel distance of the stage 5 isnormally restricted to a scope within which the interference isprevented from occurring between the stage 5 and the prober unit 7. Inthe case where a particle is at a position beyond the maximum traveldistance, it becomes difficult to find the particle. Accordingly, theinterference-prevention mechanism 6 (shown in FIG. 1) is installed tothe stage 5, and the prober unit 7, respectively.

FIGS. 4, 5 each show details, and an operation with respect to theinterference-prevention mechanism 6. The interference-preventionmechanism 6 is made up of two components, that is, a prober-unit sideinterference-prevention member 21 disposed on the prober base 10, and astage side interference-prevention member 22 disposed on the stage 5.The prober-unit side interference-prevention member 21 is movable on theprober base 10, and the prober unit 7 is disposed on the prober-unitside interference-prevention member 21. Now, when the stage 5 is movedin the direction of an arrow shown in FIG. 4 (leftward in the directionparallel to the plane of the figure), a leading-end member 23 of thestage side interference-prevention member 22 protrudes from the outerside of the stage 5, so that the leading-end member 23 interferes withthe prober mechanism system 14 before the stage 5 interferes with theprober mechanism system 14. At this point in time, the leading-endmember 23 interferes with the prober-unit side interference-preventionmember 21, whereupon the prober-unit side interference-prevention member21 is pushed by the leading-end member 23 to be moved backward of theprober base 10 (leftward in the direction parallel to the plane of thefigure).

FIG. 5 shows a state of the prober-unit side interference-preventionmember 21 that is pushed by the leading-end member 23 to be therebymoved backward of the prober base 10. The prober unit 7 fixedlyinstalled to the prober-unit side interference-prevention member 21, aswell, is concurrently moved due to the movement of the prober-unit sideinterference-prevention member 21, so that the interference between thestage 5 and the prober unit 7 can be avoided.

FIG. 6 shows details of the prober-unit side interference-preventionmember 21. There is provided a base (a slide-base lower-side member 24)serving as a rail for causing the prober-unit sideinterference-prevention member 21 to slide in such a way as to bemovable on the prober base 10, and a slide-base upper-side member 25 isprovided on the top of the slide-base lower-side member 24. Theslide-base upper-side member 25 has a groove formed along the shape ofthe rail, and the groove is fitted over the rail to thereby combine theslide-base lower-side member 24 with the slide-base upper-side member25.

Further, the slide-base upper-side member 25 is provided with a post 28around which a spring 27 is wound, and a support bar 26 for fixing thepost 28 against the slide-base upper-side member 25, as shown in aright-side figure of FIG. 6. Upon the leading-end member 23 collidingwith the slide-base upper-side member 25, the slide-base upper-sidemember 25 is moved backward on the rail serving as a guide of theslide-base lower-side member 24. When the stage 5 stops, the leading-endmember 23 as well stops, whereupon the slide-base upper-side member 25is fixedly held owing to a balance kept between a pushing force of theleading-end member 23 and a restoring force of the spring 27. For thespring 27, selection is made of a spring having an appropriate springconstant such that the restoring force will be substantially equivalentin magnitude to the pushing force of the leading-end member 23.

Next, when the stage 5 is moved in the reverse direction, theleading-end member 23 is moved away from the slide-base upper-sidemember 25, whereupon the slide-base upper-side member 25 is moved on therail in the reverse direction, thereby returning to the originalposition by the agency of the restoring force of the spring 27. Amaximum distance at which interference is preventable is dependent onthe length of the leading-end member 23, and a distance between theslide-base upper-side member 25 and the support bar 26. The maximumdistance is decided such that the travel distance of the stage 5 cancover the whole region on the specimen holder 4. With the adoption of amechanism described as above, the travel distance of the stage 5 cancover the whole region of the specimen holder 4 without causing theinterference between the stage 5 and the prober unit 7.

Upon finding of a target particle as a result of movements of the stage,a voltage is applied to the probe to thereby cause the probe to comeinto contact with a target substance (step 7). In this case, the targetparticle is moved toward the center of the SEM image, and subsequently,the voltage is applied to the probe to thereby cause the probe to moveto the vicinity of the particle. The stage 5, instead of the probe, maybe moved in the Z-axis direction to thereby cause the probe to come intocontact with the target substance. In this connection, in the case ofmoving the probe, the SEM image is brought into focus on a side of themechanism, adjacent to the target particle, whereas the SEM image isbrought into focus on a side of the mechanism, adjacent to the probe, inthe case of moving the stage, thereby causing respective movements to bemade such that the SEM images on the respective sides to fall within afocal length.

Now, there is described hereinafter control of the polarity of thevoltage applied to the probe at the time of causing the target particleto be electrostatically adsorbed. In the case of adsorbing a targetsubstance by use of an electrostatic force, if the voltage applied tothe probe is identical in polarity to an electric charge of the targetsubstance, a repulsion force instead of an adsorption force will occur,and therefore, adsorption will not take place. Accordingly, it isnecessary to control the polarity of the voltage applied to the probe incontradistinction from a charged state of the target substance. For thisreason, with the present embodiment, the prober-unit power supply 15 isprovided with a polarity-changeover switch, thereby rendering itpossible to change the polarity of the voltage applied to the probe incontradistinction from the charged state of a particle 30.

FIG. 7 shows details of control of the voltage applied to the tip of theprobe. A value of a voltage applied from the prober-unit power supply 15to a probe 20 is controlled by the controller 31. Meanwhile, theprober-unit power supply 15 is provided with a changeover switch 29, sothat the voltage applied to the probe 20 can be changed over in polaritybetween plus/minus according to a control signal from the controller 31.Because changeover in polarity between plus/minus is implemented bymanual operation undertaken by the user of the apparatus, an operationscreen where the polarity of the voltage applied to the probe 20 is setor changed is displayed on the GUI shown on the monitor 13 in the caseof the SEM according to the present embodiment. This display operationis implemented by means of the operation PC 1.

The specimen chamber 1 is filled up with a gas in order to avoidcontamination of the particle with air. Accordingly, if an excessivelyhigh voltage is applied to the probe 20, the danger of an electricaldischarge will occur. With the SEM according to the present embodiment,there is therefore pre-measured a value of the applied voltage at whichthe electrical discharge does not occur against an assumed degree ofvacuum, as an operation condition of the SEM, and respective values ofvoltages applied to the probe, corresponding to the respective degreesof vacuum are tabulated to be stored in a memory of the controller 31.The controller 31 controls a value of each voltage applied to the probe20 in accordance with a table 32 shown in FIG. 7. Furthermore, thecontroller 31 controls the needle valve 11 to thereby control the flowrate of the gas in the gas-supply piping 36. For example, in the casewhere a pressure of the gas-supply source undergoes an abrupt variation,and so forth, the controller 31 takes a reading of the flow meterassociated with the needle valve 11 to thereby control a degree ofopening/closing of the needle valve 11 so as to preclude occurrence ofthe electrical discharge at a value of a voltage presently being appliedto the probe. Otherwise, the controller 31 compares a set degree ofvacuum in the chamber 1 with the table 32, thereby causing a voltageapplied to the probe 20 to be automatically adjusted to a voltage valueat which the electrical discharge does not occur. Thus, if thecontroller 31 controls the needle valve 11 in association with thevoltage applied to the probe 20, this will enable safe adsorption of thetarget substance without the risk of the occurrence of the electricaldischarge.

In step 8, there is executed determination on adsorption. Upon theadsorption of a target particle to the probe, there occurs a change inbrightness of the SEM image, and therefore, the determination on theadsorption is executed by visual check of the change in brightness.Unless the change in brightness can be confirmed, an operation revertsto the step 7 to thereby repeat an operation in the step 8 again.

If the adsorption of the target particle to the probe is confirmed, theprobe with the target particle adsorbed thereto is moved to apredetermined position by moving the stage (step 9). When the probe ismoved, there arises a possibility that the particle is dislodged due tovibration, and so forth, and therefore, the stage is moved with thevoltage applied to the probe, in as-applied state.

Upon the probe being moved to the predetermined position, the voltageapplied to the probe is cut off, and the particle is dislodged from theprobe (step 10).

FIG. 9 shows a layout of the prober unit 7, in detail. FIG. 9 is a topview of the specimen chamber 1 as seen from the electron optical tube 2.Prober units 34, 35 are disposed in such a way as to be orthogonal toeach other against the electron optical tube 2, being adjusted such thatthe respective probes of the probers are aligned with the center line ofthe electron optical tube 2. This operation is carried out in the steps4, 5, described in FIG. 2, respectively. If the electrostatic adsorptionof the particle 30 to the probe 20 can be confirmed on the SEM screen17, the stage 5 is moved to thereby transport the particle 30 to apredetermined position. After the stage 5 is moved to a predeterminedposition, the prober unit 7 is moved to the vicinity of the specimenholder 4, whereupon the voltage applied to the probe is cut off by useof the operation PC 16. Upon confirmation that the particle 30 is placedat the predetermined position, the probe 20 is removed away from thespecimen holder 4, whereupon the operation is completed. As a result ofprocessing described as above, it becomes possible to cause the particleto be adsorbed to the probe by the agency of the electrostaticadsorption in the low vacuum SEM before being transported.

LIST OF REFERENCE SIGNS

-   1 specimen chamber-   2 electron optical tube-   3 gas amplification detector (ESED electrode)-   4 specimen holder-   5 stage-   6 interference-prevention mechanism-   7 prober unit-   8 first amplifier-   9 infrared camera-   10 prober base-   11 needle valve-   12 monitor for the infrared camera-   13 monitor-   14 prober mechanism system-   15 prober-unit power supply-   16 operation PC-   17 SEM screen-   18 tip of a probe 1-   19 tip of a probe 2-   20 probe-   21 prober-unit side interference-prevention member-   22 stage side interference-prevention member-   23 leading-end member-   24 slide-base lower-side member-   25 slide-base upper-side member-   26 support bar-   27 spring-   28 post-   29 changeover switch-   30 particle-   31 controller-   32 vacuum degree—voltage value table-   33 prober unit 1-   34 prober unit 2-   35 gas-supply piping-   38 second amplifier

The invention claimed is:
 1. A scanning electron microscope providedwith a specimen chamber for storing a target substance, and a vacuumpump for evacuating the specimen chamber into a vacuum, the scanningelectron microscope comprising: a gas introduction pipe for introducinga gas into the specimen chamber evacuated into a vacuum by use of thevacuum pump; a electron optical tube for detecting an ion current, theion current being generated by amplifying a secondary electron, or areflection electron, emitted from the target substance, upon irradiationof the target substance in the specimen chamber with a primary electronbeam, in the gas; a probe for adsorbing the target substance; aprobe-movable mechanism for causing the probe to be moved inside thespecimen chamber; and a power supply unit for applying a voltage forgenerating an electrostatic adsorption force to the probe; wherein thepower supply unit applies the voltage for electrostatically adsorbingthe target without an electrical discharge against a degree of vacuum inthe specimen chamber.
 2. The scanning electron microscope according toclaim 1, wherein the gas introduction pipe comprises a valve foradjusting a flow rate of the gas flowing inside the gas introductionpipe.
 3. The scanning electron microscope according to claim 1, furthercomprising: a valve for adjusting a flow rate of the gas flowing insidethe gas introduction pipe; and a control means for automaticallyadjusting a voltage from the power supply unit to be applied to theprobe, and the valve.
 4. The scanning electron microscope according toclaim 3, wherein the control means is provided with a table listingrespective degrees of vacuum in the specimen chamber, and respectivevalues of voltages applied to the probe, at the respective degrees ofvacuum.
 5. The scanning electron microscope according to claim 4,wherein the control means is connected to an operation PC to therebyenable automatic control by operating GUI from the operation PC.
 6. Thescanning electron microscope according to claim 1, wherein the powersupply unit is capable of changing over the polarity of the voltageapplied to the probe.
 7. The scanning electron microscope according toclaim 1, wherein a plurality of the probes can be installed to theprobe-movable mechanism.
 8. The scanning electron microscope accordingto claim 1, further comprising: a specimen holder with a targetsubstance placed thereon; and a stage for causing the specimen holder tobe moved, wherein the probe-movable mechanism and the stage are eachprovided with an interference-prevention mechanism for preventing aninterference between the probe-movable mechanism and the stage.
 9. Thescanning electron microscope according to claim 8, wherein theinterference-prevention mechanism installed on a side adjacent to theprobe-movable mechanism is a member installed on the probe-movablemechanism so as to be movable thereon.
 10. The scanning electronmicroscope according to claim 9, wherein the interference-preventionmechanism installed on a side adjacent to the probe-movable mechanism isprovided with a spring, and a support member for fixing the springagainst the interference-prevention mechanism.